Vehicles may employ powertrain systems that generate propulsion power from an internal combustion engine (engine) in conjunction with a non-combustion motor such as one or multiple high-voltage electric motor/generators. The high-voltage electric motor/generator(s) draw electric power from an on-vehicle high-voltage electric energy storage device, e.g., a high-voltage battery, via a high-voltage electric power circuit. The high-voltage battery may be charged during vehicle operation, and/or may be charged using electric power sourced from an off-vehicle electric source.
Operating circumstances such as vehicle parking for an extended period of time may result in discharging of the high-voltage battery to a charge state that is insufficient for starting the engine. Similarly, a system fault may result in discharging of the high-voltage battery to a charge state that is insufficient for starting the engine. Starting the engine is desirable.